1. Field of the Invention
The present invention relates to a method of manufacturing a rotation sensor and a structure of such a rotation sensor, and more particularly, it relates to a method of manufacturing a rotation sensor to be used for a wheel speed sensor for an automobile or the like and a structure of such a rotation sensor.
2. Description of the Background Art
In general, a wheel speed sensor is known as a sensor for detecting the wheel speed of an automobile or the like. High airtightness is required for such a wheel speed sensor, in consideration of the environment of its application. In general, therefore, an internal detecting portion (sensor element) of the wheel speed sensor is inserted in a case of stainless steel or the like, and an opening of this case is sealed with resin. This structure is disclosed in Japanese Patent Laying-Open No. 63-235811 (1988), for example. FIG. 10 is a sectional structural diagram showing the conventional wheel speed sensor disclosed in this Japanese Laying-Open Publication. Referring to FIG. 10, a sensor element, which is formed by a magnet 102, a coil 103, a magnetic pole piece 104, a bobbin 105 and the like is housed in a case 108 of stainless steel or the like in this conventional wheel speed sensor (rotation sensor). The interior of the case 108 is molded with resin by a first resin portion 109a to a portion close to an opening 112 of the case 108. Both ends of a thin wire that is wound on the coil 103 are soldered to lower portions 106a of terminals 106, while these lower portions 106a are covered with the first resin portion 109a.
A bracket 111 for fixing the rotation sensor is bonded to an outer side of the case 108 by brazing. Forward end portions 113 of an output wire 107 are joined by caulking to upper portions 106b of the terminals 106. A pipe-shaped output wire protecting member 110 is provided around a portion of the output wire 107 that is close to the forward end portions 113. The opening 112 of the case 108 and the upper portions 106b of the terminals 106 are covered with a second resin portion 109b. The second resin portion 109b is molded under a high temperature and a high pressure, so that the opening 112, the output wire protecting member 110 and the surface of the bracket 111 facing forward the output wire 107 are in close contact with the second resin portion 109b respectively.
In the conventional rotation sensor shown in FIG. 10, however, it is difficult to reduce the product cost since the case 108 is made of a high-priced metal such as stainless steel.
In general, therefore, there has been proposed a system of covering the entire sensor element with resin without employing the case 108 of stainless steel or the like. In such a system, however, the sensor element is disadvantageously moved or displaced by the pressure that is applied for molding the exterior of the sensor element with resin.
In order to prevent such a disadvantage, there has generally been proposed a method of exposing a part of a bobbin, which is one of components forming a sensor element to the exterior of a resin portion and fixing or holding the sensor element through the exposed part of the bobbin. This method is disclosed in Japanese Utility Model Laying-Open No. 61-181321 (1986), for example. FIG. 11 is a sectional structural diagram showing a rotation sensor which is disclosed in this Japanese Utility Model Laying-Open Publication. Referring to FIG. 11, the conventional rotation sensor is so structured that an end 214a of a bobbin 214 is exposed from a resin portion 220 entirely covering a sensor element. Due to this structure, it is possible to fix or hold the sensor element by the end 214a of the bobbin 214 during the molding of the resin portion 220. In the structure shown in FIG. 11, the sensor element is formed by a ferrite magnet 212, a spacer 213, a pole piece 211, the bobbin 214, and a coil 215.
In the conventional structure shown in FIG. 11, however, water permeates through a clearance between the end 214a of the bobbin 214 and the resin portion 220 in actual use, although the sensor element can be prevented from movement during the molding of the resin portion 220. As the result, insulation resistance in the interior is deteriorated, which reduces the performance. As to permeation of water through a clearance between the pole piece 211 and the resin portion 220, on the other hand, it is possible to readily improve waterproofness or water-tightness by inserting an O-ring around the pole piece 211 or the resin portion 220.
Further, a method of fixing or holding a sensor element by small pins during resin molding is also known in general, as another method of preventing the sensor element from movement during the resin molding. In this method, however, holes are formed in the resin portion where it receives the pins that are employed in the resin molding. Although the holes are filled up by resin potting or the like in a later step, water still permeates through the filled-up portions of the holes, to cause a problem which is similar to that in the structure shown in FIG. 11. Namely, insulation resistance in the interior is deteriorated, which reduces the performance by permeation of water into the interior of the resin portion (sensor element) Further, the operation for filling up the holes disadvantageously increase the number of steps.